Chapter 4.Materials Handling

4.1 Characterization of Source Emissions 4-1

4.2 Emissions Estimation: Primary Methodology 4-1

4.3 Demonstrated Control Techniques 4-4

4.4 Regulatory Formats 4-5

4.5 Compliance Tools 4-7

4.6 Sample Cost-Effectiveness Calculation 4-7

4.7 References 4-9

4.1Characterization of Source Emissions

Inherent in operations that use minerals in aggregate form is the handling and transfer of materials from one process to another (e.g., to and from storage). Outdoor storage piles are usually left uncovered, partially because of the need for frequent material transfer into or out of storage. Dust emissions occur at several points in the storage cycle, such as material loading onto the pile, disturbances by strong wind currents, and loadout from the pile. The movement of trucks and loading equipment in the storage pile area is also a substantial source of dust. Dust emissions also occur at transfer points between conveyors or in association with vehicles used to haul aggregate materials

4.2Emissions Estimation: Primary Methodology1-14

This section was adapted from Section13.2.4 of EPA’s Compilation of Air Pollutant Emission Factors (AP42). Section13.2.4 was last updated in January 1995.

The quantity of dust emissions from aggregate storage operations varies with the volume of aggregate passing through the storage cycle. Emissions also depend on the age of the pile, moisture content, and proportion of aggregate fines. When freshly processed aggregate is loaded onto a storage pile, the potential for dust emissions is at a maximum. Fines are easily disaggregated and released to the atmosphere upon exposure to air currents, either from aggregate transfer itself or from high winds. However, as the aggregate pile weathers the potential for dust emissions is greatly reduced. Moisture causes aggregation and cementation of fines to the surfaces of larger particles. Any significant rainfall soaks the interior of the pile, and then the drying process is very slow.

Table 4-1 summarizes measured moisture and silt content values for industrial aggregate materials. Silt (particles equal to or less than 75 micrometers [μm] in diameter) content is determined by measuring the portion of dry aggregate material that passes through a 200-mesh screen, using ASTM-C-136 method.1

Total dust emissions from aggregate storage piles result from several distinct source activities within the storage cycle:

Loading of aggregate onto storage piles (batch or continuous drop operations).
Equipment traffic in storage area.
Wind erosion of pile surfaces and ground areas around storage piles (see Chapter9).
Loadout of aggregate for shipment or for return to the process stream (batch or continuous drop operations).

Either adding aggregate material to a storage pile or removing it usually involves dropping the material onto a receiving surface. Truck dumping on the pile or loading out from the pile to a truck with a front-end loader are examples of batch drop operations. Adding material to the pile by a conveyor stacker is an example of a continuous drop operation.

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Table 4-1. Typical Silt and Moisture Contents of Materials at Various Industriesa

Industry / No. of
facilities / Material / Silt content (%) / Moisture content (%)
No. of
samples / Range / Mean / No. of
samples / Range / Mean
Iron and steel production / 9 / Pellet ore / 13 / 1.3-13 / 4.3 / 11 / 0.64-4.0 / 2.2
Lump ore / 9 / 2.8-19 / 9.5 / 6 / 1.6-8.0 / 5.4
Coal / 12 / 2.0-7.7 / 4.6 / 11 / 2.8-11 / 4.8
Slag / 3 / 3.0-7.3 / 5.3 / 3 / 0.25-2.0 / 0.92
Flue dust / 3 / 2.7-23 / 13 / 1 / – / 7
Coke breeze / 2 / 4.4-5.4 / 4.9 / 2 / 6.4-9.2 / 7.8
Blended ore / 1 / – / 15 / 1 / – / 6.6
Sinter / 1 / – / 0.7 / 0 / – / –
Limestone / 3 / 0.4-2.3 / 1.0 / 2 / ND / 0.2
Stone quarrying and processing / 2 / Crusted limestone / 2 / 1.3-1.9 / 1.6 / 2 / 0.3-1.1 / 0.7
Various limestone products / 8 / 0.8-14 / 3.9 / 8 / 0.46-5.0 / 2.1
Taconite mining and processing / 1 / Pellets / 9 / 2.2-5.4 / 3.4 / 7 / 0.05-2.0 / 0.9
Tailings / 2 / ND / 11 / 1 / – / 0.4
Western surface coal mining / 4 / Coal / 15 / 3.4-16 / 6.2 / 7 / 2.8-20 / 6.9
Overburden / 15 / 3.8-15 / 7.5 / 0 / – / –
Exposed ground / 3 / 5.1-21 / 15 / 3 / 0.8-6.4 / 3.4
Coal-fired power plant / 1 / Coal (as received) / 60 / 0.6-4.8 / 2.2 / 59 / 2.7-7.4 / 4.5
Municipal solid waste landfills / 4 / Sand / 1 / – / 2.6 / 1 / – / 7.4
Slag / 2 / 3.0-4.7 / 3.8 / 2 / 2.3-4.9 / 3.6
Cover / 5 / 5.0-16 / 9.0 / 5 / 8.9-16 / 12
Clay/dirt mix / 1 / – / 9.2 / 1 / – / 14
Clay / 2 / 4.5-7.4 / 6.0 / 2 / 8.9-11 / 10
Fly ash / 4 / 78-81 / 80 / 4 / 26-29 / 27
Misc. fill materials / 1 / – / 12 / 1 / – / 11
a References 1-10. ND = no data.

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The quantity of particulate emissions generated by either type of drop operation, expressed as a function of the amount of material transferred, may be estimated using the following empirical expression:11

Metric Units

English Units

where:

E = emission factor

k = particle size multiplier (dimensionless)

U = mean wind speed (meters per second, m/s, or miles per hour, mph)

M = material moisture content (%)

The particle size multiplier in the equation, k, varies with aerodynamic particle size range. For PM10, k is 0.35.11 There are two sources of fugitive dust associated with materials handling activities, namely particulate emissions from aggregate handling and storage piles, which typically consists of loader and truck traffic around the storage piles, and fugitive dust associated with the transfer of aggregate by buckets or conveyors. The PM2.5/PM10 ratios for these two sources of fugitive dust are 0.1 and 0.15, respectively.12 In general, particulate emissions from loader and truck traffic around the storage piles predominates over particulate emissions from transfer of aggregate by buckets or conveyors. Equation 1 retains the assigned quality rating of A if applied within the ranges of source conditions that were tested in developing the equation; see table below. Note that silt content is included, even though silt content does not appear as a correction parameter in the equation. While it is reasonable to expect that silt content and emission factors are interrelated, no significant correlation between the two was found during the derivation of the equation, probably because most tests with high silt contents were conducted under lower winds, and vice versa. It is recommended that estimates from Equation1 be reduced one quality rating level if the silt content used in a particular application falls outside the following range:

Ranges of Source Conditions for Equation 1
Silt content
(%) / Moisture content
(%) / Wind speed
m/s / mph
0.44 - 19 / 0.25 - 4.8 / 0.6 - 6.7 / 1.3 - 15

For Equation 1 to retain the quality rating of A when applied to a specific facility, reliable correction parameters must be determined for the specific sources of interest. The field and laboratory procedures for aggregate sampling are given in Reference 3. In the event that site-specific values for correction parameters cannot be obtained, the appropriate mean values from Table41 may be used, but the quality rating of the equation is reduced by one letter.

For emissions from trucks, front-end loaders, dozers, and other vehicles traveling between or on piles, it is recommended that the equations for vehicle traffic on unpaved surfaces be used (see Chapter6). For vehicle travel between storage piles, the silt value(s) for the areas among the piles (which may differ from the silt values for the stored materials) should be used.

Worst-case emissions from storage pile areas occur under dry, windy conditions. Worst-case emissions from materials-handling operations may be calculated by substituting into the equation appropriate values for aggregate material moisture content and for anticipated wind speeds during the worst-case averaging period, usually 24 hours. A separate set of nonclimatic correction parameters and source extent values corresponding to higher than normal storage pile activity also may be justified for the worst-case averaging period.

4.3Demonstrated Control Techniques

Watering and the use of chemical wetting agents are the principal means for control of emissions from materials handling operations involving transfer of bulk minerals in aggregate form. The handling operations associated with the transfer of materials to and from open storage piles (including the traffic around piles) represent a particular challenge for emission control. Dust control can be achieved by: (a)source extent reduction (e.g., mass transfer reduction), (b) source improvement related to work practices and transfer equipment such as load-in and load-out operations (e.g., drop height reduction, wind sheltering, moisture retention)), and (c)surface treatment (e.g., wet suppression).

In most cases, good work practices that confine freshly exposed material provide substantial opportunities for emission reduction without the need for investment in a control application program. For example, loading and unloading can be confined to leeward (downwind) side of the pile. This statement also applies to areas around the pile as well as the pile itself. In particular, spillage of material caused by pile load-out and maintenance equipment can add a large source component associated with traffic-entrained dust. Emission inventory calculations show, in fact, that the traffic dust component may easily dominate over emissions from transfer of material and wind erosion. The prevention of spillage and subsequent spreading of material by vehicles traversing the area is essential to cost-effective emission control. If spillage cannot be prevented because of the need for intense use of mobile equipment in the storage pile area, then regular cleanup should be employed as a necessary mitigative measure.

Fugitive emissions from aggregate materials handling systems are frequently controlled by wet suppression systems. These systems use liquid sprays or foam to suppress the formation of airborne dust. The primary control mechanisms are those that prevent emissions through agglomerate formation by combining small dust particles with larger aggregate or with liquid droplets. The key factors that affect the degree of agglomeration and, hence, the performance of the system are the coverage of the material by the liquid and the ability of the liquid to “wet” small particles. There are two types of wet suppression systems—liquid sprays which use water or water/surfactant mixtures as the wetting agent and systems that supply foams as the wetting agent.

Liquid spray wet suppression systems can be used to control dust emissions from materials handling at conveyor transfer points. The wetting agent can be water or a combination of water and a chemical surfactant. This surfactant, or surface-active agent, reduces the surface tension of the water. As a result, the quantity of liquid needed to achieve good control is reduced.

Watering is also useful to reduce emissions from vehicle traffic in the storage pile area. Continuous chemical treating of material loaded onto piles, coupled with watering or treatment of roadways, can reduce total particulate emissions from aggregate storage operations by up to 90%.13, 14

Table 4-2 presents a summary of control measures and reported control efficiencies for materials handling that includes the application of a continuous water spray at a conveyor transfer point and two control measures for storage piles.

Table 4-2. Control Efficiencies for Control Measures for Materials Handling

Control measure / PM10
control
efficiency / References/comments
Continuous water spray at conveyor transfer point / 62% / The control efficiency achieved by increasing the moisture content of the material from 1% to 2% is calculated utilizing the AP-42 emission factor equation for materials handling which contains a correction term for moisture content.
Require construction of 3-sided enclosures with 50% porosity for storage pile / 75% / Sierra Research, 2003.15 Determined through modeling of open area windblown emissions with 50% reduction in wind speed and assuming no emission reduction when winds approach open side.
Water the storage pile by hand or apply cover when wind events are declared / 90% / Fitz et al., April 2000.16

4.4Regulatory Formats

Fugitive dust control options have been embedded in many regulations for state and local agencies in the WRAP region. Regulatory formats specify the threshold source size that triggers the need for control application. Example regulatory formats for several local air quality agencies in the WRAP region are presented in Table 4-3. The website addresses for obtaining information on fugitive dust regulations for local air quality districts within California, for Clark County, NV, and for Maricopa County, AZ, are as follows:

· Districts within California: www.arb.ca.gov/drdb/drdb.htm

· Clark County, NV: www.co.clark.nv.us/air_quality/regs.htm

· Maricopa County, AZ: http://www.maricopa.gov/envsvc/air/ruledesc.asp

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Table 4-3. Example Regulatory Formats for Materials Handling

Control Measure / Goal / Threshold / Agency /
Establishes wind barrier and watering or stabilization requirements and bulk materials must be stored according to stabilization definition and outdoor materials covered / Limit visible dust emissions to 20% opacity / SJVAPCD Rule 8031 11/15/2001
Best available control measures: wind sheltering, watering, chemical stabilizers, altering load-in/load-out procedures, or coverings / Prohibits visible dust emissions beyond property line and limits upwind/downwind PM10 differential to 50 µg/m3 / SCAQMD Rule 403 12/11/1998
Watering, dust suppressant (when loading, stacking, etc.); cover with tarp, watering (when not loading, etc.); wind barriers, silos, enclosures, etc. / Limit VDE to 20% opacity; stabilize soil / For storage piles with >5% silt content, 3ft high, >/=150 sq ft; work practices for stacking, loading, unloading, and when inactive; soil moisture content min 12%; or at least 70% min for optimum soil moisture content; 3 sided enclosures, at least equal to pile in length, same for ht, porosity </=50% / Maricopa County Rule 310 04/07/2004
Watering, clean debris from paved roads and other surface after demolition / Stabilize demolition debris and surrounding area; establish crust and prevent wind erosion / Immediately water and clean-up after demolition / Maricopa County Rule 310 04/07/2004
Utilization of dust suppressants other than water when necessary; prewater; empty loader bucket slowly / Prevent wind erosion from piles; stabilize condition where equip and vehicles op / Bulk material handling for stacking, loading, and unloading; for haul trucks and areas where equipment op / Maricopa County Rule 310 04/07/2004

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